1. Field of the Invention
The present invention relates generally to the production of aluminum by fused bath electrolysis. In particular, it relates to cathode elements suitable for use in electrolytic cells intended for production of aluminum.
2. Description of Related Art
The cost of energy is an important concern when analyzing the operating costs of aluminum reduction plants. Consequently, a reduction in the specific energy consumption of electrolytic cells is very important for these plants. The specific consumption of a cell is equal to the energy consumed by the cell to produce one metric ton (tonne) of aluminum. It is expressed in kWh/t and, for a constant current efficiency, is directly proportional to the electrical voltage at the terminals of the electrolytic cell.
The electrical voltage of an electrolytic cell can be sub-divided into several voltage drops, namely (i) the anode voltage drop, (ii) the voltage drop in the bath, (iii) the electrochemical voltage, (iv) the cathode voltage drop and line losses. The cathode voltage drop depends on the electrical resistance of the cathode element that includes a cathode block made of a carbonaceous material and one or several metal connecting bars.
The materials from which the cathode blocks are made have changed over time such that the electrical resistance to current passing through the blocks has been getting lower and lower. As such, there are increased currents passing through the cells, while a constant cathode voltage drop is maintained.
In the 1970s, cathode blocks were made of anthracite (amorphous carbon). This material offered a fairly high electrical resistance. Faced with the needs of plants to increase their current intensity in order to increase their production, these blocks were progressively replaced with so-called “semi-graphite” blocks (containing between 30% and 50% of graphite) starting from the 1980s, then by so-called “graphite” blocks containing 100% graphite grains but whose binder between these grains remained amorphous. Since the graphite grains of these blocks have a low electrical resistance, the blocks present a lower electrical resistance to current passing through them and consequently, for constant intensity, the cathode voltage drop is reduced.
Finally, the most recent block types are so-called “graphitized” blocks. A high temperature graphitization heat treatment is carried out on these blocks, increasing the electrical conductivity of the block by graphitization of the carbon.
At the same time as these above improvements were being implemented to reduce the electrical resistance of materials, the current used in aluminum reduction plants was increased. This increase in current consequently increased the plant's production (for constant current efficiency, the number of tonnes of metal produced by a cell is proportional to the intensity of the current that passes through it). Since the cathode voltage drop Uc is equal to the product of the cathode resistance Rc and the intensity I of the current circulating in the cathode (Uc=Rc×I), the cathode voltage drops remained high, typically about 300 mV.
Furthermore, changes to the properties of cathode blocks have led to the emergence of new problems such as, for example, erosion of cathodes. For example, it has been observed that as the quantity of graphite contained in cathode blocks increases, a block becomes more sensitive to erosion problems at the head of the block. The current density is not distributed uniformly over the entire width of the pot, and there is a peak current density at each end of the block, on the surface of the cathode. This peak current density causes local erosion of the cathode, which is particularly marked when the block is rich in graphite. These very high erosion areas can limit the life of the pot, which is a major economic problem for an aluminum reduction plant.
It is known that the cathode voltage drop Uc can be reduced by using composite connection bars including a steel part and a part made of a metal with an electrical conductivity higher than steel, usually copper. Examples of patents include French patent application FR 1 161 632 (Pechiney), U.S. Pat. No. 2,846,388 (Pechiney), U.S. Pat. No. 3,551,319 (Kaiser) and international application WO 02/42525 (Servico).
It is also known from international applications WO 01/63014 (Comalco) and WO 01/27353 (Alcoa), that copper inserts can be used to improve the distribution of current along the cathode block. These documents teach to enclose a copper insert in the steel connection bar and to confine the insert inside the cell in order to reduce conduction of heat out of the cell.
However, these solutions are, first of all, expensive because copper is more expensive than steel and the copper quantities involved may be high. In the most frequently used technologies, the number of bars per electrolytic pot is usually between 50 and 100. Therefore the extra cost associated with the presence of copper components increases very quickly.
Furthermore, even the known revised configurations are not fully satisfactory. These configurations cause reductions in the global cathode voltage drop (in other words including the voltage drop in the bar) on the order of 50 mV, which is too low to justify the additional investment costs, and produce relatively high peak current densities at the head of the block, namely more than about 12 kA/m2.
Therefore the applicants tried to find satisfactory solutions to the drawbacks of prior art, and particularly to the problem of specific consumption.